1. Field of the Invention
The present invention relates to a technique of adequately changing over the compression ratio, the air-fuel ratio, and the boost status of an internal combustion engine.
2. Description of the Related Art
An internal combustion engine, which has a small in size but is capable of outputting a relatively large power, is widely used as power sources of various transportations including automobiles, ships and boats, and aircraft and as power source of various stationary equipment and machinery. The internal combustion engine compresses the air-fuel mixture, makes the compressed air-fuel mixture subjected to combustion in a combustion chamber, and converts the pressure of the combustion into a mechanical power to be output. Diverse techniques have been proposed as the results intensive research and development to enhance the conversion efficiency into the mechanical power (the thermal efficiency) and the maximum output of the internal combustion engine.
A method of varying the compression ratio is one technique developed to enhance the thermal efficiency of the internal combustion engine. As is generally known, the internal combustion engine has a theoretically optimum compression ratio, which gives the highest thermal efficiency. In the actual internal combustion engine, however, an increase in compression ratio raises the potential for the occurrence of abnormal combustion called knocking. In order to prevent the occurrence of such knocking, a smaller value than the theoretically optimum value is generally set to the compression ratio. The potential for the occurrence of knocking depends upon the driving conditions of the internal combustion engine. The technique of varying the compression ratio close to the optimum value according to the driving conditions in a knocking-free range allows for an improvement in thermal efficiency of the internal combustion engine.
Boosting is a technique developed to increase the maximum output of the internal combustion engine. The boosting technique pressurizes the air and supplies the pressurized air to the internal combustion engine. This ensures supply of a large quantity of the air and allows for combustion of a large quantity of the fuel at once, thus increasing the output of the internal combustion engine.
The method of varying the compression ratio with boosting can simultaneously enhance the maximum output and the thermal efficiency of the internal combustion engine. Boosting, however, raises the potential for the occurrence of knocking. Execution of boosting under the condition of the high compression ratio is thus not desirable, because of the higher potential for the occurrence of knocking. One technique has been proposed to adequately execute boosting while changing over the compression ratio by taking into account these factors (for example, see Japanese Patent Laid-Open Gazette No. 63-120830).
Another proposed technique to improve the thermal efficiency of the internal combustion engine makes a lean air-fuel mixture, which has a smaller rate of the fuel to the air, subjected to combustion. Still another proposed technique changes-over the air-fuel ratio of the air-fuel mixture with boosting to simultaneously enhance the maximum output and the thermal efficiency of the internal combustion engine (for example, see Japanese Patent Laid-Open Gazette No. 63-159642).
No method, however, has been proposed to attain simultaneous and adequate changeover of the settings of the boost status, the compression ratio, and the air-fuel ratio, so as to effectively enhance the thermal efficiency and the maximum output of the internal combustion engine.